The present invention relates generally to the field of micro-electromechanical actuators and more particularly to an apparatus and method for eliminating the non-linearity associated with sensing the capacitance which is associated with the operation of micro-electromechanical actuators and for achieving balance when sensing so that no net effect results from the sensing.
Developments in micro-electromechanical system (MEMS) have facilitated exiting advancements in the field of sensors, accelerometers, pressure sensors, micro-machines (microsized pumps and motors) and control components in high definition TV displays and spatial light modulators and other actuators.
Micro-mechanical actuators may have an active element in a thin metallic membrane movable through the application of a DC electrostatic field. The upper contact of the actuator includes a 0.3-millimeter aluminum or gold membrane suspended across polymer posts. Surface micromachining undercuts the post material from beneath the membrane, releasing it to be actuate. The suspended membrane typically resides, in one example 0.4-micrometers, above the substrate surface. On the substrate surface, a bottom contact includes an exemplary 0.7-micrometer gold or aluminum, first metal layer. On top of this the metal layer is positioned a thin dielectric layer, typically 1,000 xc3x85 of silicon nitride.
In the unactuated state, the membrane actuator exhibits a high impedance due to the air gap between the bottom and top plates. Application of a DC potential between the upper and lower metal plates causes the thin upper membrane to deflect downwards due to the electrostatic attraction between the plates. When the applied potential exceeds the pull-in voltage of the actuator, the membrane deflects into an actuated position. In this state, the top membrane rests directly on the dielectric layer and is capacitively coupled to the bottom plate. The capacitive coupling causes the actuator to exhibit a low impedance between the two switch contacts. The ratio of the on and off impedances of the switch is determined by the on and off capacitances of the switch in the two actuating states.
Another use for the actuator with an reflective surface is to tilt the actuator about an axis for use as a mirror. These mirrors can be used in optical devices. Additionally, the top plate includes a pivot point so that approximately half of the top membrane can pivot in one direction while the other half of the top membrane under the bottom plate can pivot in an opposite direction.
A problem with capacitance coupling devices is that capacitance varies as a non linear function with the respect to the distance between the parallel plates being sensed. Additionally, the net electrostatic force created by the sensing of the capacitive devices causes an offset and a gain error which in most cases is a highly undesired effect. In MEMS devices or any other electrostatic system or capacitance that is being sensed between the plates, the capacitance is a non-linear function of the distance between the plates. When sensing the change in capacitance, a non-linear result with respect to the positional information of the moveable plate is obtained. This often causes undesirable results or increased computation to remove the effect in these types of systems.
The present invention provides a sensing technique that electrostatically balances the device at a frequency that can be set higher than the mechanical frequency of the device that is being sensed and thus create a net zero movement in terms of sensing. By sensing the inverse of the capacitance of the actuator, the sensed voltage is an indication of the distance between the plates and the relationship between the sensed voltage and the capacitance is linear eliminating the undesired effect. Additionally, the present invention balances the sensing so that no effect due to the sensing itself is created. Thus it is possible to move a relatively small distance between the plates when the voltages are large.